In 1929, Paul Galvin, the head of Galvin Manufacturing Corporation, invented the first car radio. As the first car radios were not available from car makers, consumers had to purchase the radios separately. Lacking modern solid state electronics, these early radios were quite bulky as they were formed using vacuum tubes.
Since the introduction of this first car radio over 75 years ago, car audio systems have undergone a significant evolution. The movement to add more than just a basic radio to a car largely originated on the west coast of the United States in the late 1970's. Several early manufacturers and audio enthusiasts, such as the founder of Rockford Fosgate, Jim Fosgate, led this movement and began building audio amplifiers to run on the standard voltage in automotive electrical systems.
Today, unlike 75 years ago, auto manufacturers routinely include audio systems as standard equipment with modern vehicles. A stock car audio system refers to exactly what the manufacturer specified when constructing the car. These original factory components are referred to as Original Equipment Manufacturer (OEM) components. A custom car audio installation involves changing and/or adding “after-market” components, including anything from the upgrade of the radio/cd player to a full-blown customization of a car based around delivering exceptional sound quality or volume from audio equipment.
High-end audio systems typically include component speakers that comprise of a matched tweeter, mid-range and woofer set. These component pairs are available in two speaker and three speaker combinations, and include a cross-over which limits the frequency range that each component speaker must handle. In addition, a subwoofer(s) is provided for low frequency music information. Amplifiers boost the music signals to drive the speakers.
The most common and familiar piece of audio equipment is the radio/tape player/CD player, which is generically described as a head-unit. Since their creation, car audio head-units have generally comprised of self-contained units. The controls to operate these head-units were placed directly on the head-units. Further, these head-units typically included self-contained modular audio components such as the radio, cassette player, or CD player. As such, the head-unit has proved a highly popular component that a consumer could remove and upgrade with an after-market item that had greater functionality and quality. With the removal and replacement of the OEM head-unit, a consumer could then upgrade the car amplifier and speakers.
The human ear is capable of hearing frequencies from 20 Hz to 20 kHz. A device capable of handling frequencies from 20 Hz to 20 kHz is referred to as a full range device. An audio signal that possesses frequency information ranging from 20 Hz to 20 kHz is referred to as having full range frequency information. These different frequencies are combined to create sound. For example, a bird chirping may create frequencies around 10,000 Hz, while the human voice is around 3,000 Hz. The sound of a door slamming may lie closer to 200 Hz. These are only some examples of different sounds and their frequencies.
Music is comprised of many frequencies. Ideally, the perfect reproduction of sound would have a full range flat frequency response. A flat frequency response is one where all of the frequencies have the same amplitude or level. While a flat frequency response is desirable, it is possible to manipulate the frequency response of the sound signal to create a unique sound field for a specific vehicle. For example, many manufacturers will use the OEM head unit to attenuate low frequencies because the speakers cannot reproduce those signals accurately without sound distortion.
However, after-market head-units can offer a consumer a music frequency response without frequency or amplitude conditioning. No modifications or filtering are performed on the music signal information with an after-market head-unit, thereby allowing the reproduction of the signal as the artist intended. Consequently, replacement of the OEM head-unit is one of the most common ways to upgrade a car audio system.
The advancement of consumer electronics has enabled auto manufacturers to greatly enhance the features offered to consumers in automobiles. These features range from head-unit controls on the steering wheel and factory alarms, all the way to voice recognition, navigation, and integrated video systems. Many, if not all of these features are integrated into the OEM head-unit. The high level of integration of the OEM head-unit with the rest of the car electrical system in modern vehicles presents a problem for a consumer who wishes to install high fidelity after-market components. If the OEM head-unit is removed, these additional features are either lost or require expensive adapters to function. Sometimes, it is actually not possible to remove the OEM head-unit and still allow the car to function as designed. Further, replacing OEM amplifiers and speakers with high quality after-market items generally requires replacement of the OEM head-unit. As such, the introduction of highly integrated OEM head-units by auto manufacturers presents a significant problem.
One currently known method of addressing this problem is through the use of a device called a line-level converter. These devices convert a high-voltage level signal to a low voltage level signal. The line level converter is placed between an OEM amplifier and an after-market amplifier and after-market speakers. The line-level converter receives the high voltage signal from the OEM amplifier that would originally get transmitted to an OEM speaker, reduces it to a low voltage level line signal, and feeds it to an after-market amplifier. The after market amplifier then increases the signal and transmits it to after-market speakers. This conversion is an adequate solution to this problem if the audio content leaving the OEM head-unit has the same frequency response as that which an after-market head-unit would provide. However, typically the OEM head-units do not provide a flat frequency response, but rather typically provide a frequency response that is inferior to the flat frequency response an after-market amplifier and speakers are capable of supporting. This conditioned signal formed by the OEM head-unit that is highly integrated with the car electrical system is therefore an ongoing problem when it is not possible to remove the OEM head-unit. It is therefore highly desirable to develop an audio system that can produce a flat audio response from a factory OEM head-unit that conditions a signal.